Moderate AQI levels were reported in the Northeast and Pacific Northwest states, Louisiana and California, as shown in EPA's animation. The Code Yellow AQI levels in the Northeast are linked to PM2.5, as satellite AOD retrievals from NASA's MODIS sensor in Terra show high values coast of New England. This aerosol has been seen along a frontal boundary system for the past 2-3 days, according to NOAA's Hazard Mapping System Fire and Smoke Product.
In California, the Code Yellow AQI is associated with ozone concentrations. Satellite retrievals of NO2 concentrations (image from KNMI) were high over this region. NO2 is a ozone precursor.